The heart is generally divided into four chambers, two atrial chambers and the two ventricular chambers. As the heart beats, the atrial chambers and the ventricular chambers of the heart go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart.
The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells are contracting to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chambers to fill the ventricular chambers. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated. Control over the timing and order of the atrial and ventricular contractions during the cardiac cycle is critical for the heart to pump blood efficiently. Efficient pumping action of the heart requires precise coordination of the contraction of individual cardiac muscle cells.
Implantable cardiac pacemakers have been successfully used to maintain control over the timing and order of the cardiac cycle. In its simplest form, the cardiac pacemaker is an electrical circuit in which a battery provides electricity that travels through a cardiac lead to a cardiac electrode and into the heart causing a contraction, and back to the battery to complete the circuit. Cardiac electrodes are typically implanted within or adjacent one cardiac chamber. This allows for cardiac signals to be sensed predominately from that chamber and for electrical energy pulses to be delivered to that chamber. For example, tip electrodes on transvenous leads are typically implanted in the apex of the right ventricular chamber or at or near the atrial appendage of the right atrium. Because the tip electrode is implanted completely within one cardiac chamber, electrical pulses provided through the tip electrode stimulate the chamber in which the electrode is implanted. So, for example, a pacing pulse delivered to an atrial electrode implanted in the atrial appendage stimulates the atria to contract. Likewise, a pacing pulse delivered to a ventricular electrode implanted in the right ventricle apex stimulates the ventricles to contract.
A current trend in cardiac rhythm management devices, also referred to as implantable pulse generator systems, is to implant cardiac electrodes in and/or through the coronary sinus vein. The coronary sinus vein drains venous blood from the coronary arteries into the right atrium. The coronary sinus vein also allows access to cardiac locations that are adjacent to either the left atrium and/or the left ventricle, where access to the left ventricle is typically gained through the great cardiac vein which is coupled to the coronary sinus vein. As such, the coronary sinus vein is an avenue for accessing, sensing and providing stimulation to different sites of the heart.
One difficulty encountered when using transvenous electrodes implanted within the coronary sinus is that electrical pulses delivered to capture the atrium can also capture the ventricles, or visa versa. This situation is referred to as “cross capture.” Cross capture arises from the fact that the coronary sinus is generally located between the atrial chambers and the ventricular chambers along the anterior groove. When transvenous electrodes are positioned in this region of the heart it is possible for electrical pulses intended to stimulate the atrial chamber to instead, or in addition to, stimulate the ventricular chamber. This situation is undesirable, as hemodynamic efficiency is adversely effected when the ventricles contract too soon with respect to the atrial chambers. Thus, a need exists for a reliable way of preventing unintentional cross capture pacing.